Active fan flutter control

ABSTRACT

A flutter control system for a turbomachine fan includes a plurality of fan case sensors located at a fan case of the turbomachine and configured to sense passing of blade tips of a fan of the turbomachine. A controller is operably connected to the plurality of fan case sensors. A variable fan area nozzle actuator is operably connected to the controller, such that the variable fan area nozzle actuator urges a change in fan nozzle area in response to data from the plurality of fan case sensors indicating flutter or near flutter conditions. A method of flutter control for a turbomachine fan includes sensing a blade tip passing of a plurality of fan blades. Data from a plurality of fan case sensors is compared to a threshold and a fan exit area is changed based on the comparison to dampen flutter of the plurality of fan blades.

BACKGROUND OF THE INVENTION

The subject matter disclosed herein relate to turbine engines. Morespecifically, the subject disclosure relates to control of flutter offan blades of turbine engines.

Turbine engines include a fan section having a number of airfoils, orfan blades, extending from a hub. In large turbine engines these fanblades can exceed five feet in length. The length and ratio of length tochord of fan blades leaves them susceptible to flow induced vibration,also known as flutter. Fan blade flutter can lead to structural damageto and failure of the fan blade, and liberation of the fan blade fromthe hub which results damage to components and potentially failure ofthe turbine engine. Fan blades typically are susceptible to flutterduring certain operation conditions of the turbine engine. Typically,flutter is avoided by avoiding those operating conditions which may leadto flutter. This, however, places undesirable restrictions on theoperation of the turbine engine.

BRIEF DESCRIPTION OF THE INVENTION

According to one aspect of the invention, a flutter control system for aturbomachine fan includes a plurality of fan case sensors located at afan case of the turbomachine and configured to sense passing of bladetips of a fan of the turbomachine. A controller is operably connected tothe plurality of fan case sensors. A variable fan area nozzle actuatoris operably connected to the controller, such that the variable fan areanozzle actuator urges a change in fan nozzle area in response to datafrom the plurality of fan case sensors indicating flutter or nearflutter conditions.

According to another aspect of the invention, a method of fluttercontrol for a turbomachine fan includes sensing a blade tip passing of aplurality of fan blades via a plurality of fan case sensors disposed ata fan case of a turbomachine. Data from the plurality of fan casesensors is compared to a threshold and a fan exit area is changed basedon the comparison to dampen flutter of the plurality of fan blades.

These and other advantages and features will become more apparent fromthe following description taken in conjunction with the drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

The subject matter, which is regarded as the invention, is particularlypointed out and distinctly claimed in the claims at the conclusion ofthe specification. The foregoing and other features, and advantages ofthe invention are apparent from the following detailed description takenin conjunction with the accompanying drawings in which:

FIG. 1 is a schematic of an exemplary embodiment of an active fluttercontrol system; and

FIG. 2 is a schematic of another exemplary embodiment of an activeflutter control system.

The detailed description explains embodiments of the invention, togetherwith advantages and features, by way of example with reference to thedrawings.

DETAILED DESCRIPTION OF THE INVENTION

Shown in FIG. 1 is a schematic of an embodiment of an active fluttercontrol system 10. The system includes a plurality of tip timing probes12 located in a fan case 14 of a turbomachine 16. The tip timing probes12 are located to observe arrival timing of a plurality of fan blades 18fixed to a fan shaft 20 as the plurality of fan blades 18 rotate about afan axis 22. In the embodiment of FIG. 1, three tip timing probes 12 areutilized, located in the fan case 14 substantially to monitor passing ofa leading edge 24, trailing edge 26, and mid-chord 28 of the pluralityof fan blades 18. The tip timing probes 12 monitoring the leading edge24 and trailing edge 26 are utilized to determine fan blade 18 twist.The tip timing probes 12 at mid-chord 28 monitor tip timing to determineflex of the fan blades 18. Even though three tip timing probes 12 areutilized in the embodiment of FIG. 1, it is to be appreciated that otherquantities of tip timing probes 12, for example, five or six tip timingprobes 12, may be used. Further, groups of tip timing probes 12 may bepositioned radially around the fan case 14, for example at 12 o'clock, 4o'clock, and 8 o'clock positions. This allows for collection of more tippassing data and correlation and/or verification of data when taken atmultiple locations around the fan case 14.

Together, the information from the tip timing probes 12 is communicatedto a full authority digital engine control (FADEC) 30. The FADEC 30compares the passing timing of the fan blades 18 to a threshold, todetermine if a fan blade 18 is approaching a flutter condition or isactively fluttering. Based on the comparison, the FADEC 30 sendscommands to a variable fan area nozzle (VFAN) actuator 32. The VFANactuator 32 drives a VFAN 34 to change a fan exit area 36. The change tofan exit area 36 ensures that sufficient back pressure is applied to thefan blades 18 to dampen out flutter as measured by the tip timing probes12.

Another embodiment of a flutter control system 10 is shown in FIG. 2. Inthis embodiment, a plurality of radio frequency (RF) probes 38 locatedin the fan case 14 to observe arrival timing of the plurality of fanblades 18. As with the embodiment of FIG. 1, three RF probes 38 areutilized, located in the fan case 14 substantially to monitor passing ofthe leading edge 24, trailing edge 26, and mid-chord 28 of the pluralityof fan blades 18. The RF probes 38 monitoring the leading edge 24 andtrailing edge 26 are utilized to determine fan blade 18 twist. The tiptiming probes 12 at mid-chord 28 monitor tip timing to determine flex ofthe fan blades 18.

A plurality of strain gauges 40 are arranged along a span 42 of the fanblade 18 and are connected to a radio frequency identification (RFID)tag 44 at the fan blade 18. In FIG. 2, the strain gauges 40 are arrangedsubstantially linearly along the span 42, but this arrangement is merelyexemplary. Any appropriate arrangement of strain gauges 40, for example,placement of strain gauges 40 at known high stress points, iscontemplated within the present scope. The RFID tag 44 communicates withone or more of the RF probes 38 to relay data from the plurality ofstrain gauges 40 to the FADEC 30. The FADEC 30 utilizes data from theplurality of strain gauges 40 and the RF probes 38 to determine flutterstatus of the fan blades 18. Utilizing the plurality of strain gauges 40and the RFID tag 44 provides the FADEC 30 with more data to moreaccurately determine the flutter status of the plurality of fan blades18. As in the embodiment of FIG. 1, the FADEC 30 is linked to the VFANactuator 32 to change the fan exit area 36 to mitigate flutter of theplurality of fan blades 18.

While the invention has been described in detail in connection with onlya limited number of embodiments, it should be readily understood thatthe invention is not limited to such disclosed embodiments. Rather, theinvention can be modified to incorporate any number of variations,alterations, substitutions or equivalent arrangements not heretoforedescribed, but which are commensurate with the spirit and scope of theinvention. Additionally, while various embodiments of the invention havebeen described, it is to be understood that aspects of the invention mayinclude only some of the described embodiments. Accordingly, theinvention is not to be seen as limited by the foregoing description, butis only limited by the scope of the appended claims.

1. A flutter control system for a turbomachine fan comprising: aplurality of fan case sensors disposed at a fan case of the turbomachineand configured to sense passing of blade tips of a fan of theturbomachine; a controller operably connected to the plurality of fancase sensors; and a variable fan area nozzle actuator operably connectedto the controller, such that the variable fan area nozzle actuator urgesa change in fan nozzle area in response to data from the plurality offan case sensors indicating flutter or near flutter conditions.
 2. Thesystem of claim 1, further comprising at least one blade sensor disposedon a blade of the fan.
 3. The system of claim 2, wherein the at leastone blade sensor is operably communicative with the controller.
 4. Thesystem of claim 3, further comprising a radio frequency identificationtag disposed at the blade and at least one radio frequency sensordisposed at the fan case.
 5. The system of claim 2, wherein the at leastone blade sensor is at least one strain gauge.
 6. The system of claim 1,wherein the plurality of fan case sensors is three fan case sensors. 7.The system of claim 1, wherein the plurality of fan case sensors are aplurality of tip timing probes.
 8. The system of claim 1, wherein thecontroller is a full authority digital engine control (FADEC).
 9. Amethod of flutter control for a turbomachine fan comprising: sensing ablade tip passing of a plurality of fan blades via a plurality of fancase sensors disposed at a fan case of a turbomachine; comparing datafrom the plurality of fan case sensors to a threshold; and changing afan exit area based on the comparison to dampen flutter of the pluralityof fan blades.
 10. The method of claim 9, further comprising:communicating the data from the plurality of fan case sensors to acontroller; comparing the data to a threshold at the controller; andsending a command from the controller to a variable fan area nozzleactuator to change the fan exit area.
 11. The method of claim 10,further comprising: collecting strain data via at least one fan bladesensor disposed at the plurality of fan blades; communicating the straindata to the controller; and comparing the strain data to a strainthreshold.
 12. The method of claim 11, further comprising communicatingthe strain data to the controller via a radio frequency identificationtag.
 13. The method of claim 9, further comprising: evaluating a twistof the plurality of fan blades via the plurality of fan case sensors;and evaluating a flex of the plurality of fan blades via the pluralityof fan case sensors.